Backwards compatibility

If you upgrade from an older version of Umbraco then legacy NotFoundHandlers listed here 'should' still work - The request pipeline contains a ContentFinderByNotFoundHandlers IContentFinder that will attempt to execute the functionality of any legacy NotFoundHandlers, but it's recommended to move your custom request handling logic to IContentFinders.

Implementing 404 not found properly

Custom 404's are now handled within a custom IContentFinder by setting the Is404 property to true for the PublishedContentRequest processed by the IContentFinder or by registering the IContentFinder as a ContentLastChanceFinder - Using an IContentFinder for Custom 404s

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